This invention relates to signal converting apparatus. More specifically, it relates to an apparatus for converting an analog or digital signal which varies in a non-linear relationship to the value of a particular variable to a digital signal which varies in a linear relationship.
The measurement and display of variables such as temperature, pressure, density, etc., are frequently desired in the form of a digital display. In carrying out these measurements, it is sometimes necessary to use sensing elements which provide an electrical signal (such as voltage or current) the magnitude of which varies in response to the quantity being measured. Thus, for example, thermocouples may be used to measure temperature in that they produce an output voltage which is related to temperature. That voltage can be suitably processed and displayed in digital form to give a direct indication of the magnitude of that particular variable. The thermocouple, however, as with many other sensors, produces a voltage output which is non-linearly related to the magnitude of the quantity being measured. Thus, a plot of the output voltage of the thermocouple against the temperature being measured by the thermocouple is not a straight line over the normal range of interest.
Various means have been utilized for linearizing the digital signals provided, upon conversion of the analog output of the thermocouple, for example, to digital form so as to provide a digital display which is directly readable as temperature in degrees Fahrenheit or Centigrade. An example of one form of linearization is to be found in U.S. Pat. No. 3,686,665 to Elias et al, assigned to the assignee of this invention. In the disclosure of that patent there is set forth a method which utilizes an identification cycle which identifies the instantaneous value of the input, identifies a particular segment of the function on which that instantaneous value of the input is located, and identifies one or more count modifiers corresponding to the identified segment of the function. A register is then cleared and the readout cycle is begun during which a digital count accumulates in the register and is modified by one or more count modifiers to give the digital output in linearized form. That system has as a disadvantage, the requirement of using alternating identification and readout cycles, thus requiring a longer time period to accomplish the linearization than is the case with the present invention.
Other forms of linearization which have been used include those which utilize the count of pulses out of a frequency divider as a basis for modifying the division factor. Such systems are inherently more difficult to program for different ranges and different primary measuring elements.